Hi Dmitry,

On Tue, 31 Dec 2024 17:29:52 +0200
Dmitry Baryshkov [off-list ref] wrote:

On Tue, Dec 31, 2024 at 11:40:04AM +0100, Luca Ceresoli wrote:

quoted

This driver implements the point of a DRM pipeline where a connector allows
removal of all the following bridges up to the panel.

The DRM subsystem currently allows hotplug of the monitor but not preceding
components. However there are embedded devices where the "tail" of the DRM
pipeline, including one or more bridges, can be physically removed:

 .------------------------.
 |   DISPLAY CONTROLLER   |
 | .---------.   .------. |
 | | ENCODER |<--| CRTC | |
 | '---------'   '------' |
 '------|-----------------'
        |
        |               HOTPLUG
        V              CONNECTOR
   .---------.        .--.    .-.        .---------.         .-------.
   | 0 to N  |        | _|   _| |        | 1 to N  |         |       |
   | BRIDGES |--DSI-->||_   |_  |--DSI-->| BRIDGES |--LVDS-->| PANEL |
   |         |        |  |    | |        |         |         |       |
   '---------'        '--'    '-'        '---------'         '-------'

 [--- fixed components --]  [----------- removable add-on -----------]

This driver supports such a device, where the final segment of a MIPI DSI
bus, including one or more bridges, can be physically disconnected and
reconnected at runtime, possibly with a different model.

The add-on supported by this driver has a MIPI DSI bus traversing the
hotplug connector and a DSI to LVDS bridge and an LVDS panel on the add-on.
Hovever this driver is designed to be as far as possible generic and
extendable to other busses that have no native hotplug and model ID
discovery.

This driver does not itself add and remove the bridges or panel on the
add-on: this needs to be done by other means, e.g. device tree overlay
runtime insertion and removal. The hotplug-bridge gets notified by the DRM
bridge core after a removable bridge gets added or before it is removed.

The hotplug-bridge role is to implement the "hot-pluggable connector" in
the bridge chain. In this position, what the hotplug-bridge should ideally
do is:

 * communicate with the previous component (bridge or encoder) so that it
   believes it always has a connected bridge following it and the DRM card
   is always present
 * be notified of the addition and removal of the following bridge and
   attach/detach to/from it
 * communicate with the following bridge so that it will attach and detach
   using the normal procedure (as if the entire pipeline were being created
   or destroyed, not only the tail)
 * instantiate two DRM connectors (similarly to what the DisplayPort MST
   code does):
   - a DSI connector representing the video lines of the hotplug connector;
     the status is always "disconnected" (no panel is ever attached
     directly to it)
   - an LSVD connector representing the classic connection to the panel;
     this gets added/removed whenever the add-on gets
     connected/disconnected; the status is always "connected" as the panel
     is always connected to the preceding bridge  

I'd rather have just a single connector. MST connectors can be added and
gone as there is fit, so should be your LVDS panel-related connector.

The plan we discussed at LPC 2024 is to eventually get rid of the first
connector (see "Roadmap and current status" in the cover letter), so
you can consider this legacy code. However the current implementation
won't work without this connector, so it is still there for the time
being. Pointing this out in a note in the commit message of this patch
would probably be useful to avoid future misunderstanding, so I'm
adding one for v6.

Thanks for reviewing!

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com